Thin film transistor (TFT) technology is the basis for high-resolution, high-performance liquid crystal display (LCD) screens. TFT technology provides the best resolution of the various flat-panel display technologies that are currently available. Advanced TFT technology is based on polycrystalline silicon.
Polycrystalline silicon may be formed using laser recrystallization techniques such as excimer laser annealing (ELA). In ELA, a high-power laser beam is scanned over the surface of a substrate that is coated with amorphous silicon. The amorphous silicon is heated, melts and then recrystallizes into polycrystalline silicon.
A more recently introduced laser recrystallization technique is 2-shot sequential lateral solidification. In a typical application, a two-dimensional mask pattern is imaged upon the amorphous silicon film by an imaging lens. Only the irradiated areas melt and recrystallize. By repetitive irradiation of different areas, the whole substrate silicon film can be recrystallized. The resulting quality of the polycrystalline silicon film exceeds that of ELA processed material in various parameters.
TFT technology requires extremely high quality processes and high process speeds. These requirements place great demands on the parameters of the imaging of an SLS system. It is particularly important for the focal plane of the imaging lens to be precisely determined. One conventional approach to focus determination in SLS systems involves visual inspection of test patterns. The test patterns are formed in a substrate while varying the z-position of the focal plane of the image. The substrate is then removed from the SLS system and visually examined by a human operator under a bright source of light. The operator looks for differences in the surface texture of the substrate. These differences are called protrusions. The protrusions are formed most sharply and clearly at points in the test pattern where the focal position of the image was adjusted correctly. By manually inspecting the surface texture of the substrate, a human operator can select a focal plane and calibrate the SLS system accordingly.
There are a number of challenges associated with this conventional approach to calibration. First, in order for a human operator to visually inspect a substrate, the substrate must be removed from the associated SLS system. This removal costs time and money and significantly increases the risk of damage to a substrate. The removal is particularly problematic in the case of larger substrates such as G4 and G5 substrates. In addition, because the necessary visual inspection must be conducted by a human operator, the operator must be given special training. Even with such training, the results produced by human operators tend to be somewhat inconsistent and subjective.